Point density and transparent background
If point density is visible in RGB (set the viewer channel to "Color" only) it means that pixels in transparent areas should emit light, despite they don't occlude the background.

That's perfectly fine for an associated alpha image, and it means it can be conmposited straight away with an alpha over operation in the compositor.

Just go to the compositor and use an alpha over node to composite your plate over a solid background (black or gray should work) and check the result.

If that worked it means your plate is perfectly fine for compositing, although the viewer won't show the emissions in transparent pixels.

This is caused by a longstanding bug (sadly still present in 2.8) that makes the viewer pre-multiply images (that are already associated, resulting in a double up), making transparent emissions disappear.
They are there, but the broken viewer doesn't show them.